Generally, throttle bodies restrict the amount of air inducted into an internal combustion engine, based on input from various factors such as accelerator pedal position, thus controlling the engine output. A valve, commonly a butterfly valve, is typically employed to control the amount of air flow. At idle, the valve is closed and air can only flow either through a bypass valve, if so equipped, or through small openings left around the valve for this purpose. Under certain engine idle conditions, more air flow through the throttle body is needed in order to increase the idle speed to allow for smooth, stable idle. Some examples of such conditions are starting and running when the engine is cold, and when running heavy load auxiliary equipment, such as vehicle air conditioning.
Many prior art designs use an air by-pass solenoid valve mounted in a separate bore that bypasses the throttle plate. However, this design can cost more than is desirable and creates a possible air leak path through the bypass valve, making the amount of air flow difficult to accurately calibrate, particularly for small capacity engines, (e.g., less than one liter in displacement). With the bypass tube, the solenoid valve cannot independently determine its position without a separate position sensor, thus further raising the likelihood of inaccuracies in the system.
Other prior art designs modulate the throttle valve itself to control idle air flow in order to avoid the use of a bypass valve. However, they typically employ a gear train, used to maximize throttle valve position resolution, and a linear motor that operates the throttle valve via a push rod. This gear train helps to allow for adequate motor torque build-up, without requiring an excessively large motor to drive the system adequately, and helps to overcome the inherent limitation on a typical motor that the running torque is less than the holding torque. Nevertheless, the gear train slows down the response time and can be relatively complex and expensive, with several failure modes possible. This is the trade off of mechanical advantage versus response time, (motor size versus gearing).
Further, it is desirable that the idle control assembly have a failure mode which drops the throttle plate back to low idle if the motor fails, rather than being stuck where it is if the motor fails at high idle. Also, preferably, an idle control system is employed that will not restrict the smooth movement of the accelerator pedal at off-idle conditions and will allow for a linearized increase in air flow in order to aid in engine calibration.
Therefore, it is desirable to have a simple, thus inexpensive and likely more reliable, design that still has a fast response time and high resolution with minimal power use, while still minimizing the size of the components within the assembly. The same desire is also true of a port throttle design wherein each intake port includes its own air control valve, as opposed to a design with just one air control valve in a single throttle body.